The structure of human and other mammalian sperm chromatin is being studied by electron microscopy, flow cytometry and biochemical dissection. More specifically, we will investigate the presence of circular DNA in sperm nuclei, the possible subunit structure of nuclear DNA, the nature of disulfide bonding in nuclear chromatin and the resistance of nuclear DNA to in situ thermal denaturation. The data will be correlated when possible to the fertility level of the donor.